Acceleration only increasing when I press a key

I've tried to design my code so that when I'm not moving, my acceleration will decrease and when I'm moving, my acceleration will increase.

The issue is, my acceleration only increases the moment I push a key and doesn't increase while it's held and will then continue to decrease to the minimum value unless I keep pushing the movement keys.

Here are my code files;

Char Controller Class

    public class CharController : MonoBehaviour {

     public float minAcceleration, maxAcceleration;

     public float Acceleration;

     protected Rigidbody rb;

     protected Vector3 Movement = new Vector3(0, 0, 0);
     protected Vector3 OldMovement;

     protected float MoveHorizontal, MoveVertical;

     protected System.Random rand = new System.Random();

     protected virtual void Start() {
         this.rb = GetComponent<Rigidbody>();
         this.Acceleration = minAcceleration;
     }

     protected virtual void FixedUpdate() {
         this.MoveHorizontal = (float)this.rand.NextDouble();
         this.MoveVertical = (float)this.rand.NextDouble();

         this.OldMovement = Movement;
         this.Movement = new Vector3(this.MoveHorizontal, 0.0f, this.MoveVertical);

         if (!this.Movement.Equals(new Vector3(0, 0, 0))) {
             this.rb.MovePosition(this.rb.position + (this.Movement * this.Acceleration));
         } else {
             this.rb.MovePosition(this.rb.position + (new Vector3(1, 0, 1) * this.Acceleration));
         }
         this.UpdateAcceleration();
     }

     protected virtual void UpdateAcceleration() {
         if ((!this.OldMovement.x.Equals(this.Movement.x)) || (!this.OldMovement.z.Equals(this.Movement.z))) {
             if (this.Acceleration < this.maxAcceleration) {
                 this.Acceleration += (this.Acceleration / 100);
             }
         } else if (this.minAcceleration < this.Acceleration) {
                 this.Acceleration -= (this.Acceleration / 100);
             }

         if (this.Acceleration < this.minAcceleration) {
             this.Acceleration = this.minAcceleration;
         } else if(this.maxAcceleration < this.Acceleration){
             this.Acceleration = this.maxAcceleration;
         }
     }

 }

Player Controller Class;

 public class PlayerController : CharController {

     protected override void FixedUpdate() {
         this.MoveHorizontal = Input.GetAxis("Horizontal");
         this.MoveVertical = Input.GetAxis("Vertical");

         this.OldMovement = Movement;
         this.Movement = new Vector3(this.MoveHorizontal, 0.0f, this.MoveVertical);

         if (Input.GetKey(KeyCode.W) || Input.GetKey(KeyCode.A) || Input.GetKey(KeyCode.S) || Input.GetKey(KeyCode.D)) {
             if (!this.Movement.Equals(new Vector3(0, 0, 0))) {
                 this.rb.MovePosition(this.rb.position + (this.Movement * this.Acceleration));
             } else {
                 this.rb.MovePosition(this.rb.position + (new Vector3(1, 0, 1) * this.Acceleration));
             }
         }
         this.UpdateAcceleration();
     }
 }